Im boarding for about 5 years now, in total 6 weeks.
This year for the last time i will rend my gear for boarding. I want to begin with doing freestyle tricks like 180's, rails etc. just playing around with the board on the track.

personally, I have never understood an unequal duck stance (ie 18/-9, 12/-17). To me that is straight up uncomfortable.

I ride 15/15 wide stance, and couldnt be happier. easy to ride switch and do 180's, feels balanced and stable.

I hope you mean you ride 15/-15. And yes I agree that an equal duck stance feels so much better than an unequal, but I'm sure people's legs are built differently and their knees don't bend at quite the same angle. Personally I ride 12/-12.

personally, I have never understood an unequal duck stance (ie 18/-9, 12/-17). To me that is straight up uncomfortable.

I ride 15/15 wide stance, and couldnt be happier. easy to ride switch and do 180's, feels balanced and stable.

I think it might have to do with peoples hip rotation. If you have a wide stance but can't keep your knees over your toes its going to spell trouble.
You should not be forcing your knee over your toe with twisting your feet, it should come from the rotation of the hip socket.

I am naturally more flexible on one side that the other (also knew this from years of ballet and leaning how to turnout properly)
I ride +15 and -12 on the back (think I got that the right way round, at work, cant check). I have fiddled with it and tried other stance, I eventually set it to where I would bend without forcing my knees over my toes and feel comfortable riding switch.
It might be like this for other riders too?

Your stance is personal to you and one stance does not work for everyone, Im +21/-18 but it works for me. Also most knee pain comes from riding too wide for you frame, although like binding angles some people can pull it off.

Yeah there's nothing wrong with asymmetrical duck stance... When my GF was learning we set her up with -6/+15 or so, to give some forward bias. Last year we slowly moved her to -9/+9 where she's happiest.

I ride at -12/+12 and that seems to work for me.

If your focus for the week will be on freestyle, get a more freestyle oriented board. Of course you can use an all-mountain in the park, hell I use freeride boards in the park! But again, if your focus is on freestyle, get a freestyle board!